Natural Selection vs Genetic Drift: Founder Effect, Bottleneck and Gene Flow

Evolution is a change in allele frequencies in a population over generations. Five processes can change them, and each one breaks a Hardy-Weinberg assumption. The key contrast is that natural selection is nonrandom and adaptive, while genetic drift is random and can remove alleles regardless of benefit.

The five forces

Natural selection

Fitness is reproductive success, meaning the number of surviving offspring, not strength. Selection acts on phenotype, but only heritable variation changes the next generation.

• Directional: one extreme is favored, so the mean shifts.
• Stabilizing: the middle is favored and variation shrinks (human birth weight).
• Disruptive: both extremes are favored over the middle, which can lead to speciation.
• Sexual selection: traits that win mates, even at a survival cost.
• Heterozygote advantage keeps a harmful recessive allele common (sickle cell allele and malaria).

Genetic drift

Drift is a random change in allele frequency from chance, and it matters most in small populations. Over time it can fix one allele (frequency 1) and lose the other. Two classic cases:

• Bottleneck effect: a disaster cuts the population to a few survivors, whose alleles may not match the original pool.
• Founder effect: a few individuals start a new population, carrying only part of the source variation. Rare alleles can be common in the new group.

Gene flow and speciation

Gene flow makes populations more similar and reduces differences. Reproductive isolation stops it, and speciation follows. Allopatric speciation is by geographic separation, and sympatric is within one area.

Worked example

A population of 10,000 has the allele A at p = 0.6 and a at q = 0.4. A storm leaves 10 survivors who happen to carry few a alleles, and the new frequency is q = 0.1 with p = 0.9. No allele was favored by selection. This is a bottleneck, and the later genotype frequencies are q² = 0.01 aa, 2pq = 0.18 Aa, p² = 0.81 AA.

Common traps

• Calling drift adaptive. It is random.
• Thinking fitness means strength or lifespan. It is offspring contributed.
• Saying mutation drives rapid change. It supplies variation but changes frequencies slowly.
• Saying individuals evolve. Populations do.

One-minute recap

Selection is nonrandom and adaptive. Drift is random and strongest in small groups, with bottleneck and founder effect as special cases. Gene flow homogenizes populations. Mutation supplies new alleles. Nonrandom mating changes genotype, not allele, frequencies.